The earliest retroreflective sheeting had an exposed-lens construction. In such sheeting, retroreflection of light was inhibited when the lenticular surfaces of the lenses were covered with water. This problem was answered by enclosed-lens retroreflective sheeting, such as is taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), wherein the lenses, typically glass microspheres with specular reflective layers disposed behind the rear surfaces thereof, were totally embedded within a sheeting that had a flat transparent top film. This allowed incident light rays to be focused onto the specularly reflective layer irrespective of whether the front of the sheet was wet or dry. U.S. Pat. No. 3,190,178 (McKenzie) solved the same problem in a different way. That patent discloses a cellular retroreflective sheeting that is formed from (1) a base sheet comprising retroreflective elements partially embedded in a binder layer and (2) a cover sheet. The cover sheet is sealed to the base sheet along a network of interconnecting bonds to form hermetically sealed cells with protected retroreflective elements having an air-interface therein An advance upon this technique is disclosed in U.S. Pat. No. 4,025,159 (McGrath) wherein the binder material is taught to be thermoformable material that is cured in situ after being thermoformed, thereby achieving more reliable adhesion of the binder material to the cover sheet. Two common embodiments of encapsulated-element sheetings are (1) microsphere-based sheetings which typically comprise a monolayer of microspheres partially embedded in layer of binder material which is thermoformed in a grid pattern or network of intersecting bonds into contact with a substantially transparent cover film in front thereof and (2) prismatic or cube-corner retroreflective sheetings which typically comprise internally reflecting elements located on the back side of a face sheet that has a flat front face and a sealing layer located behind the elements which is sealed to the face sheet in a network of intersecting bonds.
In order for encapsulated-element retroreflective sheetings to provide desired performance it is important the cover film remain firmly bonded to the remaining portions of the sheeting in order to maintain the desired encapsulated, i.e., sealed, structure. Further, the components of the sheeting should be sufficiently durable and remain stable so as to maintain (1) desired arrangement to provide the optical functionality necessary for retroreflection, e.g., the desired air-interface, and (2) structural integrity. Many steps can be taken to achieve such durability. For instance, U.S. Pat. No. 4,637,950 (Bergeson et al.) discloses a cover film for encapsulated-element retroreflective sheetings which provides increased resistance to delamination. The aforementioned U.S. Pat. No. 4,025,159 discloses another technique for achieving increased delamination resistance in encapsulated-element retroreflective sheetings by strengthening the bond of the cover or sealing film thereto.
Because of the high visibility which retroreflective products such as sheetings can provide, they are commonly applied to many items for purposes of communication, e.g., road signs bearing information for travelers, and safety, e.g., traffic cones, clothing, life rafts, etc. In some instances, such applications may expose the sheetings to conditions or agents which may cause degradation of the sheeting, thereby impairing or even destroying, the retroreflective performance of the sheeting. For instance, sheetings used in outdoor applications should be resistant to exposure to ultraviolet radiation and moisture.
One common and demanding application of retroreflective sheetings is on construction zone markers and traffic delineators, e.g., traffic cones, barriers, etc. Such articles are typically subject to rough treatment and exposure to varied, often extreme, environmental conditions. In such applications, the articles or substrates to which the sheetings are mounted may present another risk to the retroreflective performance of the sheeting. Traffic cones, for instance, are typically made so as to be very flexible. Commonly they may be made of such materials as polyvinyl chloride which is highly plasticized in order to achieve high flexibility and impact resistance. Furthermore, in order to provide high conspicuity, traffic cones may be colored, such as with pigments or dyes, with very bright, conspicuous colors, e.g., blaze or fluorescent orange.
In order to be used on such articles, retroreflective sheetings should be impact resistant and flexible; accordingly, cover films, binder materials, etc., are typically selected to exhibit the desired properties such as abrasion resistance, flexibility, and resistance to degradation by exposure to ultraviolet radiation ("UV"). Another risk, however, is that agents within the substrate article such as plasticizers, will tend to migrate from the substrate into the sheeting, commonly also causing pigments or other agents in the substrate to penetrate the sheeting as well.
In many instances, penetration of plasticizer into a retroreflective sheeting can result in degradation of the sheeting, thereby reducing the useful life of the sheeting. For instance, the bond between the cover film and other elements of the sheeting may be weakened such that the cover film may tend to delaminate, thereby rendering the sheeting subject to impairment by moisture deposition on the surfaces of the microspheres or prismatic elements so as to destroy the desired air-interface. The plasticizer may penetrate or migrate into the sheeting and form a layer on the optical surfaces of the retroreflective elements which are supposed to have an air-interface (i.e., the front surfaces of microspheres or the surface of prismatic elements depending upon the type of sheeting), thereby impairing retroreflection just as moisture can on exposed-lens constructions. In some instances, one or more elements of the sheeting, e.g., the binder layer or prismatic elements, may be deleteriously affected such that the structural integrity of the sheeting is lost and retroreflective performance is substantially impaired or even lost, e.g., the cover film may wrinkle, or the retroreflective elements may distort in shape or orientation. Furthermore, migrating plasticizer may cause unsightly discoloration of some portion of the sheeting, e.g., the seal legs of which a white or other specified color is often desired. In many instances, when the plasticizer migrates into the sheeting it may tend to carry along other agents, e.g., colorants in the substrate such as pigments and dyes, which further impair the performance or appearance of the sheeting.